Toss to coat all the onions in the butter, reduce the heat to medium-low and let the onions saute slowly for 30 minutes, only stirring very occasionally. After 30 minutes they should be nice and caramelized.

Use the remaining tablespoon of butter to spread on one side of each slice of bread.

With the buttered side facing down, layer half the cheese slices on top of the bread.

Next lay the roast beef on top of the cheese.

Spoon some of the onions (you may have extra) on top of the roast beef.

Top the onions with the remaining cheese and then the other slice of bread with the buttered side facing up.

Transfer the sandwich to a skillet over medium heat.

Press the sandwich down using a large spatula and hold pressure on it so that the cheese melts. After about 2 minutes, flip the sandwich over and repeat. The first side should be a nice golden brown. Alternatively, you could use a panini press for this if you have one.

Once the other side is golden brown and the cheese is melted, remove from the skillet, cut in half and serve.